Changing strings is mainly a matter of sound... if the sound you want isn't there anymore, that's usually the time to change them. As far as keeping them clean between changings is pretty low-tech. I use pads from Musicians Friend just for that purpose. They are similar to the napkins in the foil from KFC. If you play a lot (everyday) use it every day. If you play like me (weekly) use it weekly. Regardless of what method you use, the idea is to get the sweat and acids your hands leave in the string grooves. I know of some players whose acid is so strong that if left uncleaned, the strings will tarnish in two days!